DESCRIPTION: The goal of this project is to investigate how the auditory system represents the pitch of complex sounds ("periodicity pitch") in the discharge patterns of central auditory neurons. Periodicity pitches in speech (voice pitch) convey information for voicing, prosody, and speaker identity, while those in music form melodies and harmonies. Striking correlates of periodicity pitch exist in patterns of interspike intervals produced by auditory nerve fibers. This project will investigate what happens to this information more centrally by recording spike trains of single neurons in the cochlear nucleus and inferior colliculus of anesthetized cats. Experiments will use a diverse set of monaural and binaural stimuli which are known to produce similar periodicity pitches. Patterns of discharge in single neurons and ensembles of neurons will be analyzed in order to identify potential neural codes for pitch. Population response profiles will be constructed from ensembles of units having the same physiologically-defined response type. Single unit and population responses in the cochlear nucleus and inferior colliculus will be compared to those at the level of the auditory nerve. Alternative neural representations for pitch based on rate, synchrony, and interspike intervals will be evaluated by comparing neural response patterns with those of human pitch judgments. Neural responses to dichotic stimuli that, through their binaural interaction, create periodicity pitches will be studied for the first time in the inferior colliculus. The study will elucidate the role of temporal information in central auditory representations, knowledge which is immediately relevant for more optimal design of auditory implants and hearing aids, communications systems, and devices for speech recognition. This knowledge is also important for understanding the role of neural timing deficits in central auditory disorders, the role of autocorrelation-like representations in audition, and, more generally, the role of neural temporal pattern codes in the brain.